878 



DIBRANCHIATE CEPHALOPODS. 



The siphuncle is marginal, enclosed in a calcareous sheath, and 

 contracted where it pierces the successive septa. Atractites, also 

 from the Alpine Trias, differs from Aulacoceras chiefly in the want 

 of lateral furrows. 



In the singular genus Xiphoteuthis, of the Lower Lias, the guard 

 is cylindrical ; the phragmacone is greatly elongated, and increases 

 very slowly in width ; and there is a narrow " pro-ostracum," which 

 is at least five times as long as the guard. 



In the Tertiary rocks, the family of the Belemnitidoe is represented 

 by the genera Belemnosis, Beloptera, and Spirulirostra. The only 

 known species of Belemnosis is found in the Eocene rocks (London 

 Clay), and has a short obtuse guard, with a terminal pore. The 

 phragmacone has horizontal septa and a marginal siphuncle. The 

 genus Beloptera is also known by a single species only, and likewise 

 occurs in the Eocene rocks. In this curious type the shell consists 

 of two conical segments joined point to point, and further united 

 by wing -like lateral expansions. The posterior portion of the 

 skeleton represents the guard, while the phragmacone is lodged in 

 the anterior portion. 



Lastly, in the genus Spirulirostra (fig. 809) the skeleton consists 

 of a triangular, pointed guard, which is hollowed out in front for the 



Fig. 809. — Spirulirostra Bellai'dii. Miocene Tertiary. 



reception of a chambered portion or phragmacone. This latter is 

 spirally bent, and the septa are pierced along its concave or ventral 

 side by a marginal siphuncle. The only known species of Spiruli- 

 rostra is found in the Miocene deposits of Italy. 



Family 3. Belemnoteuthid^e. — In this family the skeleton is 

 internal, and resembles that of the Belemnitidce. in consisting of a 

 " guard," a chambered " phragmacone," and a thin shelly " pro- 

 ostracum." The guard is, however, reduced to a thin calcareous 



